1. Field of the Invention
The present invention relates to a twin drum type continuous casting method in which a pair of cooling drums, rotating in opposite directions to each other, form a molten metal storage portion therebetween and discharge a thin metal plate downward thereof.
2. Description of the Related Art
In a twin drum type thin plate continuous casting apparatus, a pair of cooling (casting) drums, which are cooled by cooling water circulating in an interior thereof, are supported horizontally by bearings fitted to a frame so as to be parallel to maintain a casting gap which corresponds to a cast piece thickness. Above the casting gap of the cooling drums, there is formed a molten metal storage portion to which a molten metal is supplied continuously from a tundish via a nozzle.
The molten metal in the molten metal storage portion comes in contact with a surface of the cooling drums and is cooled to form a solidified shell, which in turn is led by the pair of cooling drums, which are driven to rotate in opposite directions to each other, to be discharged from the casting gap of the cooling drums as a cooled and solidified thin strip cast piece.
In the twin drum type continuous casting method for casting a cast piece as mentioned above, in order to obtain a cast piece having no defects such as surface cracks (fine cracks) etc. and having an excellent characteristics and quality, there are effectively used a shot blasting method, a photo-etching method, an electric discharge machining method, an electron beam machining method, etc., which are applied to a cylindrical surface of the cooling drum of the continuous casting apparatus so as to form a multiplicity of minute dimples uniformly or at random thereon, each said dimple having a shape of circle, oval, etc. of depth of approximately 5 to 100 .mu.m and diameter of approximately 0.1 to 1.2 mm. This is known from publications of the Japanese laid open patent application Nos. Sho 60(1985)-184449, Sho62(1987)-254953, Sho 64(1989)-83342, etc.
Also, in the Japanese laid open patent application No. Hei 6(1994)-39501, as shown in FIG. 5, there is disclosed a twin drum type thin plate continuous casting apparatus comprising a detecting device 113, 114 for detecting a surface temperature of a cast piece by sweeping thereon in the widthwise direction, a control device 112 to which a detected value of the cast piece surface temperature detecting device 113, 114 is inputted and a shot blasting device 115, 116 which, receiving a control signal from the control device 112, moves in the axial direction of the cooling drum 1, 1' to apply a shot blasting onto a surface of the cooling drum 1, 1'. Meanwhile, in FIG. 5, numeral 110, 111 designates a brush for cleaning the drum.
In the prior art twin drum type continuous casting apparatus as shown in FIG. 5, temperature of the cast piece 40 is detected so that an area where the temperature is lower than a permissible temperature is detected and the control device 112 moves a nozzle of the shot blasting device 115, 116 to a position on the cylindrical surface of the cooling drum which corresponds to said area and causes a shot blasting to operation start. Also, it is so described there that even if there occur dimple worn-out places irregularly here and there, the control device 112 aims at such places and applies the shot blasting easily.
However, if such a cooling drum as having a sufficient cooling effect of the drum surface is employed, the surface dimples wear substantially uniformly as a whole and there occur surface temperature lowered portions everywhere until it comes to a time to detect such a partial temperature lowering in the cast piece as mentioned above, so that there is a problem in that a good response cannot be attained by detection of the respective temperature lowered portions and control of the nozzle movement to the position to be shot-blasted.